Inking systems for lithographic and other types of printing presses require that some of the rollers be oscillated in the axial direction to eliminate ridging and to minimize ghosting. To accomplish this, many press designers utilize external worm drives which are well known in the art and date back to the Middle Ages. Such drives are an integral part of the press, are installed during manufacture, and have proven to be rugged and reliable.
In order to further improve print quality, additional oscillating rollers are sometimes incorporated into a press after it has been installed and operated for some time. Due to space limitations it is generally necessary for such rollers to have self-contained mechanisms for generating the oscillatory motion. However, also because of space limitations, no satisfactory arrangement has been found which, to date, utilizes the proven worm drive concept in add-on rollers which have a self-contained mechanism.
Generally, the self-contained mechanisms for generating characterized further according to the three types of cam surfaces employed: continuous single revolution barrel, continuous duplex or cross threaded, and dual discontinuous cam surfaces of opposite lead.
The most straightforward mechanism is the single barrel type where a barrel cam is mounted on the inside of the rotating roller and one or more followers are secured to the non-rotating roller shaft. Alternately, the cam can be mounted on the shaft and the follower(s) on the roller.
In the known devices, exemplified by U.S. Pat. No. 3,110,253, one cycle of axial oscillatory motion is generated for each revolution of the roller. However, at high press speeds the rapid oscillatory motion produced by this design can cause unwanted streaks in the printed product.
To correct this problem some designs have utilized gears internally and externally to reduce the relative rotational speed of cam and follower, thereby slowing down the axial oscillatory motion. U.S. Pat. No. 2,040,331 is an example of such a device where the gears are located inside the roller. U.S. Pat. No. 4,397,236, on the other hand, is an example of where the gears are located external to the roller.
The second type of device also uses a continuous cam having a multi-rotational surface. Such a cam is known as a duplex or cross-threaded cam and is exemplified by the cams disclosed in U.S. Pat. Nos. 715,902 and 4,040,682. In these designs, several revolutions of the roller are required to produce one cycle of oscillatory motion. One problem encountered with this type of prior art device is that the mechanism is prone to jam as a result of wear.
In the third type of mechanism, disclosed for example in U.S. Pat. Nos. 1,022,563 and 4,833,987, two discontinuous cam surfaces of opposite lead are employed. Oscillatory motion is provided by using two cam followers each of which alternately engages and disengages one of the cam surfaces. One problem encountered with these designs is excessive wear at high press speeds and resultant malfunctioning.
Thus, prior known internal mechanical devices have experienced problems such as mechanical wear for one reason or another. One reason for mechanical wear is that the force needed to produce the axial motion is generated at the contact point between the cam and follower. Wear can result at this point. In those designs which do not utilize gears, the relative speed of the follower is very high relative to the cam. In those designs which employ internal gears, the gears must be small enough to fit inside the roller. As a result, the gears must travel at relatively high speeds which may result in excessive wear after extended use.
Therefore, a significant problem encountered with all prior art self-contained designs for use in inking systems is poor reliability resulting from excessive mechanical wear, especially at high press speeds. Another problem with many prior art devices is that they are not compact enough to be used in certain locations in the press. A third problem with some prior art designs is that the oscillatory motion produced is not pure harmonic, i.e. is not sinusoidal.
Therefore, there presently exists a need for a self-driven oscillating roller which utilizes a worm drive mechanism compact enough to fit inside such a roller, and thus significantly reduces or avoids the aforementioned problems associated with the devices currently utilized in the art.